According to court documents, an Ohio man was arrested on Wednesday for plotting an attack on the U.S. Capitol. (Reuters)

Federal authorities on Wednesday arrested an Ohio man who espoused support for the Islamic State and who allegedly plotted to attack the U.S. Capitol in a military-style assault, according to court documents.

Christopher Lee Cornell, a 20-year-old resident of the Cincinnati area, was charged with purchasing and possessing firearms and attempting to kill U.S. government officers and employees.

Although Cornell expressed support for the Islamic State, the militant group based in Iraq and Syria, he does not appear to have any formal backing from overseas. Rather, according to an affidavit in support of a criminal complaint filed in U.S. District Court for the Southern District of Ohio, he indicated that by conducting an attack he would be “fulfilling the directives of violent jihadists.”

The arrest comes amid renewed concern about the threat of so-called homegrown terrorism and the possibility that Americans with little or no affiliations with radical groups could carry out attacks.

Cornell was taken into custody by the FBI’s Joint Terrorism Task Force. It was unclear whether he had a lawyer.

Christoper Lee Cornell, 20, was charged Wednesday with purchasing and possessing firearms and attempting to kill U.S. government officers and employees. (Butler County Sheriff's Office/Getty Images)

According to the affidavit in support of the criminal complaint, Cornell first came to the attention of the FBI when he used Twitter accounts to post statements, videos and other information indicating his support for the Islamic State and violent attacks committed by others in North America and elsewhere.

In the fall, an informant began cooperating with the FBI as a way to obtain favorable treatment in an unrelated criminal case and agreed to provide information about a person using the name Raheel Mahrus Ubaydah — Cornell’s alias on Twitter, according to the affidavit.

Cornell and the informant corresponded by instant message. At one point, according to the affidavit, Cornell wrote: “I believe that we should just wage jihad under our own orders and plan attacks and everything.”

“We already got a thumbs up from the Brothers over there and Anwar al Awlaki before his martyrdom and many others,” he allegedly wrote.

Awlaki, who was killed in a U.S. drone strike in 2011, was a propaganda leader for al-Qaeda’s franchise in Yemen, a rival group to the Islamic State.

On Oct. 17 and 18, Cornell met with the informant in Cincinnati and again talked about his support of the Islamic State and waging jihad. He said he wanted to “move” in December, according to the affidavit, but wasn’t ready to reveal his plans at their first meeting.

He then showed the informant jihadist videos on his computer as well as information on how to make pipe bombs. He had also calculated the cost for firearms and bomb ingredients and identified a nearby gun store that would sell a Smith & Wesson semiautomatic rifle, according to the affidavit.

This week, Cornell allegedly made final plans to travel to Washington to set off the bombs. Cornell purchased two semiautomatic rifles and about 600 rounds of ammunition in Ohio before he was arrested by FBI agents, according to the affidavit.

A bulletin issued by the FBI and the Department of Homeland Security to law enforcement agencies said Cornell’s activities “highlight the continued interest of US-based violent extremists to support designated foreign terrorist organizations overseas, such as ISIL, by committing terrorist acts in the United States.” ISIL is an acronym used to describe the Islamic State.

John A. Barrios, acting special agent in charge of the FBI’s Cincinnati division, said the public was not in danger during the investigation into Cornell’s activities.